Loop balance and Common Mode (CM) noise level measurements on a twisted pair in an xDSL communication system (e.g. ADSL or VDSL) are an important tool for an operator in order to qualify the susceptibility of in-house wiring in which customer premises equipment (CPE) is located to external noise sources.
For example, as part of a noise and loop impairment identification and classification scheme, the measurement of the loop imbalance contains valuable information for a service provider as the measurement reflects the susceptibility of the loop to external interference or to the likelihood of the loop itself radiating external interference. Generally, an imbalanced twisted pair with respect to ground will radiate external interference by converting differential signals to common mode signals and vice versa. These common mode signals may become a source of disturbances to other services (e.g. PLC receivers, AM and HAM radio receivers) or contribute to the increase of self-FEXT and self-NEXT for other VDSL services sharing the same cable binder. On the other hand, an imperfect twisted pair that is imbalanced to some degree with respect to ground tends to be more susceptible to external interference. Specifically, some of the common mode signals present on a twisted pair and induced by an external source (PLC transmitters, AM, HAM transmitters) will have a greater susceptibility to being converted to differential noise signals affecting the DSL capability of the imperfect twisted pair.
Imbalanced tip and ring pairs comprising a twisted pair occur as a result of cable imperfection during manufacturing (such as wire and twist variations), or more significantly during placement/installation, during which bends and impartial twisting may be introduced at both ends of the cable. Furthermore, the existence of non-twisted pairs in the drop which do not present proper imbalance with respect to earth (as opposed to a cable sheath in which twisted pairs are generally enclosed) may actually enhance an imbalance measurement. Finally, split-pair configurations in which wires of different twisted pairs are inadvertently paired to carry a DSL signal will produce an abnormally high level of imbalance. A short of either the tip or ring to ground (which may occur inadvertently) also results in an imbalance measurement.
One existing method to estimate the loop imbalance in a DSL system is described in U.S. Pat. No. 8,687,770 entitled “Systems and Methods for Performing Line Imbalance Measurement and Mitigation Based on a Common Mode Sensor.” In this method, a near-end or far-end differential mode (DM) DSL signal is transmitted and the received common mode (CM) signal measured on a CM port is correlated with it. The loss measured is representative of the imbalance of the loop, and therefore the susceptibility of this loop to convert CM signals to DM.
While useful in some applications, there are certain drawbacks with this approach. For example, it requires the transmission of an excitation signal in order to derive a metric associated to the loop imbalance, which is not always possible or feasible. As another example, it does not address how a CM sensor can be implemented for the measurement of the loop imbalance, without contributing to a degradation of the loop imbalance itself and the level of converted common mode signal into differential mode either resulting from the deterioration of the CM rejection ratio of the front end or because of the change of CM termination impedance of the loop due to the use of a CM sensor. Finally, it does not consider how to perform the calibration of such a measurement in order to cope with imperfections of the front end balance.
Therefore, there remains a need in the art for methods and apparatuses capable of characterizing CM noise and estimating loop imbalance that address these and other challenges.